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Containment of Polynitroaromatic Compounds in a Hydrogen Bonded Triarylbenzene Host Published as part of the Crystal Growth & Design virtual special issue IYCr 2014 - Celebrating the International Year of Crystallography Pratap Vishnoi, Mrinalini G. Walawalkar, and Ramaswamy Murugavel* Department of Chemistry, Indian Institute of Technology Bombay, Mumbai, India 400 076 S Supporting Information *

ABSTRACT: Co-crystallization of energetic materials has emerged as an important technique to modify their critical properties such as stability, sensitivity, etc. Using 1,3,5-tris(4′-aminophenyl)benzene (TAPB) as a novel co-crystal former, we have prepared co-crystals of 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitrophenol (TNP), and m-dinitrobenzene (mDNB). Molecular structures of the co-crystals have been determined from single crystal X-ray diffraction data. The diffraction data analysis reveals that strong intermolecular π−π interaction directs the intercalation of polynitroaromatic explosives (PNACs) between the layers of TAPB molecules, which leads to the formation of vertically overlapped -A-B-A-B- types of π-stacks. Both TNT and TNP form π-interactions with the center of TAPB with 1:1 molar ratios, while mDNB forms a complex in a 1:3 stoichiometry through stacking between peripheral rings. The crystal lattices are further stabilized through interstack hydrogen bonds (N−H···N and N−H···O) between amino groups of TAPB and nitro groups of PNACs. NMR and Fourier transform infrared spectra further provide the information about the presence of various interactions in the crystal systems. Owing to the π electron-rich nature and ease of synthesis, triphenylbenzene systems are promising host candidates for co-crystallization of PNAC analytes.

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INTRODUCTION Polynitroaromatic compounds (PNACs) such as 2,4,6trinitrotoluene (TNT), 2,4,6-trinitrophenol (TNP), 2,4-dinitrotoluene (DNT), 2,4-dinitrophenol (DNT), and dinitrobenzene (DNB) represent a class of chemical explosives containing large amounts of chemical energy stored in their molecular structures (Chart 1).1−3 These compounds rapidly undergo

oxygen and fuel components and high chemical potentials have been invented, only a few withstand the critical requirements for safe transportation and use in desired field applications. During the past decade, considerable research efforts have focused on improving the performance of existing explosives through modification of the critical properties such as shock sensitivity, density, melting point, stability, and reactivity. Recently, co-crystallization of explosive materials with nonexplosives/other explosive compounds has gained significant attention as it could effectively reduce shock sensitivity and density and improve the stability. For instance, Landenberger and Matzger modified the properties of 1,3,5,7-tetranitro1,3,5,7-tetrazacyclooctane (HMX) by co-crystallization with a wide variety of nonenergetic aromatic co-crystal formers that possess highly polarized oxygen atoms available to form hydrogen bonds with HMX ring protons.5 These results revealed a tremendous reduction in the sensitivity as compared to pure HMX, and thus it can be handled safely. The group of Matzger has also reported co-crystals of diacetone diperoxide (DADP)/1,3,5-trichloro-2,4,6-trinitrobenzene (TCTNB) and DADP/1,3,5-tribromo-2,4,6-trinitrobenzene (TBTNB) through chlorine−nitro interactions.6 The same group also

Chart 1. Molecular Structures of PNAC Compounds Discussed in the Present Studies

explosion with the generation of an enormous amount of heat, gases, and pressure in the presence of external forces such as temperature, electric shock, mechanical shock, and friction.2 Even though explosive materials have been widely used in mining, armaments, space exploration, and fireworks for a long time, the development of new explosives is relatively slow.3,4 This is attributed to the inherent safety−power contradiction where thermally stable energy materials shows low impact sensitivity or vice versa, which in fact limits their utility.4 Though several new energetic materials balanced favorably with © XXXX American Chemical Society

Received: June 29, 2014 Revised: September 10, 2014

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heated under reflux for 10 h. The solution was subsequently kept at 0 °C for crystallization. Red microcrystalline compound was obtained from the solution after 2 days. Recrystallization from acetonitrile at room temperature yielded X-ray quality single crystals of TAPB−TNT (0.025 g, 62% based on TAPB used). Mp; 160 °C. Anal. Calcd for C31H26N6O6: C, 64.35; H, 4.35; N, 14.53. Found: C, 63.45; H, 4.30; N, 13.0. 1H NMR (400 MHz, DMSO-d6, 295 K): δ 9.0 (s, 2 H, Ar−H, TNT), 7.47 (d, 3JHH = 8.68 Hz, 6 H, Ar−H, TAPB), 7.45 (s, 3 H, Ar− H, TAPB), 6.67 (d, 3JHH = 8.48 Hz, 6 H, Ar−H, TAPB), 5.21 (s, 6 H, TAPB-NH2), 2.56 (s, 3 H, Ar−CH3, TNT) ppm. 13C{1H} NMR (100 MHz, DMSO-d6, 295 K): δ 169.7, 150.8, 148.2, 145.7, 141.6, 132.9, 127.4, 122.6, 120.4, 114.2, 15.0 ppm. FT-IR (KBr diluted disc): 3378 (m), 3330 (m), 3106 (m), 2925 (m), 1607 (s), 1538 (s), 1517 (s), 1342 (s) cm−1. TAPB−TNP. TAPB (0.05 g, 0.14 mmol) and TNP (0.034 g, 0.15 mmol) was taken in methanol (20 mL) and stirred for 10 min and then kept for crystallization at room temperature. Red needle-like crystals of TAPB−TNP were obtained from the solution after 2 days (0.060 g, 73% based on TAPB used). Mp 235 °C (dec.). Anal. Calcd for C30H24N6O7: C, 62.07; H, 4.17; N, 14.48. Found: C, 62.13; H, 4.06; N, 13.73. 1H NMR (400 MHz, DMSO-d6, 295 K): δ 8.58 (s, 2 H, Ar−H, TNP), 7.66 (d, 3JHH = 8.48 Hz, 6 H, Ar−H, TAPB), 7.63 (s, 3 H, Ar−H, TAPB), 6.94 (d, 3JHH = 8.44 Hz, 6 H, Ar−H, TAPB) ppm. 13 C{1H} NMR (100 MHz, DMSO-d6, 295 K): δ 160.9, 142.7, 141.9, 141.3, 132.0, 127.9, 125.3, 124.4, 121.8, 117.4 ppm. FT-IR (KBr diluted disc): 3319 (m), 3226 (m), 1606 (b), 1512 (b), 1263 (b) cm−1. TAPB−mDNB. TAPB (0.025 g, 0.07 mmol) and mDNB (0.036 g, 0.21 mmol) was taken in methanol (20 mL). The mixture was stirred at RT for 10 min and kept for crystallization. Slow evaporation of the solvent over 24 h yielded red needle-like crystals of TAPB−mDNB (0.042 g, 70% based on TAPB used). Mp 110 °C. Anal. Calcd for C42H33N9O12: C; 58.95, H; 3.89, N; 14.73. Found: C; 59.14, H; 3.86, N; 12.65. 1H NMR (400 MHz, DMSO-d6, 295 K): δ 8.84 (t, 3JHH = 2.10 Hz, 3 H, Ar−H, m-DNB), 8.67 (dd, 3JHH = 8.20 Hz and 4JHH = 2.16 Hz, 6 H, Ar−H, m-DNB), 7.99 (t, 3JHH = 8.25 Hz, 3 H, Ar−H, mDNB), 7.48 (d, 3JHH = 8.52 Hz, 9 H, Ar−H, TAPB), 6.67 (d, 3JHH = 8.44 Hz, 6 H, Ar−H, TAPB), 5.22 (s, 6 H, TAPB-NH2). 13C{1H} NMR (100 MHz, DMSO-d6, 295 K): δ 148.4, 148.0, 141.6, 131.6, 129.4, 128.0, 127.5, 120.4, 118.6, 114.20 ppm. FT-IR (KBr diluted disc): 3473 (m), (3378 (m), 3112 (m), 1621 (m), 1516 (s), 1345 (s) cm−1. TAPB−DNP. TAPB (0.025 g, 0.07 mmol) and DNP (0.016 g, 0.071 mmol) was mixed in methanol (20 mL). The mixture was stirred at room temperature for 10 min and kept for crystallization at the same temperature. Red color compound was obtained from the solution in a week (0.030 g, 73%). 1H NMR (400 MHz, DMSO-d6, 295 K): δ 8.70 (d, 3JHH = 2.84, 3 H, Ar−H, DNP), 8.32−8.30 (dd, 3JHH = 2.92 and 3 JHH = 9.32, 3 H, Ar−H, DNP), 7.54 (t, 3JHH = 8.52, 9 H, Ar−H, TAPB), 7.20 (d, 3JHH = 9.28, Ar−H, DNP), 6.76 (d, 3JHH = 8.42, 6 H, Ar−H, TAPB) ppm. Anal. Calcd for C42H33N9O15: C; 55.82, H; 3.68; N, 13.95. Found: C, 54.37; H, 2.83; N, 13.94. Following a similar synthetic protocol, TAPB was combined with other PNAC analytes such as 2,4-dinitrophenol (DNP), pdinitrobenzene (pDNB), and m-dinitrotoluene (DNT). However, no co-crystals could be obtained in these cases, although red adducts are formed in each case; a 1:3 red complex of TAPB−DNP was isolated. X-ray Crystallography. The X-ray diffraction data for all the compounds were collected on a Rigaku Saturn 724 CCD diffractometer with a Mo−Kα radiation source (λ = 0.71075 Å) at 150 K (for the TAPB−PNAC complexes) and 100 K (for TAPB) under continuous flow of cooled nitrogen gas. All the structures were solved by direct methods using SIR-9219 and refined by full-matrix least-squares fitting on F2 using SHELX-97.20 All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were refined isotopically as rigid atoms in their idealized locations. The amino-H atoms were located from difference Fourier maps and refined independently. More details regarding crystallographic studies are given in Supporting Information. The data refinement details are provided in Table 1.

reported co-crystals of TNT with different aromatic and heterocylic nonexplosive coformers.7 A number of other groups have also reported co-crystals of explosive materials.8,9 The formation of co-crystals between two complementary chemical entities relies greatly on supramolecular bonding such as hydrogen and π−π interactions.10,11 It is discouraging that nitrated explosives are chemically inert as their chemistry is mainly defined by nonreacting nitro groups. Fortunately, PNACs are π-electron-deficient and have proven reliable to form donor−acceptor π−π interactions with electron-rich systems.11−13 It is noteworthy that typically only a few molecular systems such as naphthalenes, anthracenes, and perylenes are reported to have the capability of forming interactions with π-electron-deficient energetic materials.7,14 The scarcity of co-crystal formers necessitates investigating new molecular systems that can favorably interact with PNACs. Recently we reported that C3-symmetric triphenylbenzene compounds are π-electron-rich fluorescent systems which selectively interact with PNACs, leading to the fluorescence quenching, and serve as potential chemosensors.15 Aminosubstituted triphenylbenzene, such as 1,3,5-tris(4′aminophenyl)benzene (TAPB) has high-energy highest occupied molecular orbitals (HOMOs), which could give favorable π-stacking overlapping with low-lying lowest unoccupied molecular orbitals (LUMOs) of PNACs.15 These studies suggest that this system could also serve as an efficient co-crystal former for nitro explosives. TAPB with three amino groups on the periphery can form acid−base complexes with acidic −OH containing PNACs such as TNP and DNP. TNP acts both as an electron-acceptor and acidic ligand and forms crystalline solids with aromatic anilines16 and N-donor bases17 primarily through hydrogen-bonding and aromatic−aromatic interactions. Taking a clue from these studies, in the present work we have attempted the containment of several polynitroaromatic analytes inside the supramolecular TAPB lattice through combination of π−π stacking and hydrogen bonding interactions. In this endeavor, we were able to isolate adducts formed by TNT, TNP, and mDNB with TAPB whose details are described below.

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EXPERIMENTAL SECTION

Methods, Materials, and Instruments. Starting materials such as m-dinitrobenzene (Thomas Baker, India), 2,4,6-trinitrophenol (Loba Chemie, India), 2,4-dinitrotoluene (Sigma-Aldrich) were procured from commercial sources and used after recrystallization. 1,3,5-Tris(4′aminophenyl)benzene (TAPB) was synthesized according to a published procedure.18 The melting points were measured in glass capillaries and reported uncorrected. Elemental analyses were performed on a Thermo Finnigan (FLASH EA 1112) microanalyzer. Fourier transform infrared spectra (FT-IR) were recorded on a PerkinElmer Spectrum One Infrared spectrometer as KBr diluted pellets. Differental scanning calorimetry (DSC) was performed on Shimazdu DSC-60 instrument. NMR experiments were carried out on a Bruker 400 MHz instrument using DMSO-d6 (D, 99.9%) as solvent. The chemical shift values are relative to the deuterated solvent peaks and are given in parts per million (ppm). Abbreviations, s = singlet, d = doublet, t = triplet, and J = coupling constant have been utilized to describe the peak patterns. Caution. Polynitroaromatic compounds are sensitive to external stimuli such as mechanical shock, heat, electromagnetic radiation, static electricity, friction, etc.; therefore, it is highly advisible to handle these materials with due care. TAPB−TNT. TAPB (0.025 g, 0.07 mmol) and 2,4,6-trinitrotoluene (0.017 g, 0.08 mmol) was taken in absolute ethanol (10 mL) and B

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Table 1. Crystal Data and Structure Refinement for TAPB, TAPB−TNT, TAPB−TNP, and TAPB−mDNB

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formula formula wt temperature [K] wavelength [Å] crystal system space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] volume (Å)3 Z density (calcd) [g/cm3] absorption coeff F(000) crystal size [mm3] θ range [deg] reflection collected data (Rint) completeness to θ [%] restraints/parameters GOF on F2 R1 [I > 2σ(I)]/all data wR2 [I > 2σ(I)]/all data largest peak and hole (e·Å−3)

TAPB

TAPB−TNT

TAPB−TNP

TAPB−mDNB

C24H21N3 351.44 100(2) 0.71075 triclinic P1̅ 9.246(3) 10.328(4) 10.939(5) 76.69 (2) 86.21(2) 66.800(17) 934.0(6) 2 1.250 0.074 372 0.24 × 0.15 × 0.11 3.03−25.00 7031 3211(0.0448) 97.8 0/224 0.859 0.0452/0.0637 0.0933/0.0977 0.202, −0.263

C31H26N6O6 578.58 150(2) 0.71075 triclinic P1̅ 7.158(4) 14.331(8) 14.651(8) 114.992(9) 96.439(6) 96.452(4) 1332.1(12) 2 1.442 0.103 604 0.13 × 0.10 × 0.06 2.68−25.00 9934 4657(0.0527) 99.1 0/385 0.998 0.0553/0.0684 0.1486/0.1576 0.368, −0.268

C30H24N6O7 580.55 150(2) 0.71075 monoclinic P21/n 6.943(3) 14.503(5) 25.797(9) 90 97.480(7) 90 2575.5(17) 4 1.497 0.110 1208 0.12 × 0.10 × 0.05 2.12−25.00 14900 4482(0.0416) 98.7 0/388 1.137 0.0553/0.0683 0.1293/0.1445 0.254, −0.308

C42H33N9O12 855.77 150(2) 0.71075 triclinic P1̅ 7.431(14) 12.83(4) 20.81(7) 88.87(15) 79.84(13) 88.35(11) 1952(10) 2 1.456 0.110 888 0.06 × 0.06 × 0.04 2.53−25.00 15016 6815(0.0303) 99.3 0/568 1.120 0.0609/0.0806 0.1309/0.1436 0.164, −0.216

RESULTS AND DISCUSSION TAPB possesses a π electron-rich core built up of a triphenylbenzene core featuring an electrostatic potential that makes it an ideal co-crystal former for electron-deficient PNAC analytes. TAPB readily forms charge transfer red crystals with PNACs that have been isolated and characterized by melting point, FT-IR, and NMR spectroscopy, and single crystal X-ray diffraction. Isolation and Characterization of the Complexes. The TAPB−TNT complex is isolated as a microcrystalline product by heating an ethanolic mixture under reflux for 10 h followed by cooling to 0 °C. X-ray quality crystals have been obtained from acetonitrile solution. TAPB−TNP and TAPB−mDNB complexes have been crystallized from methanol at RT. The stoichiometry of TAPB to PNAC in crystals remained unchanged irrespective of the ratio in which the starting materials have been mixed. TAPB−TNT and TAPB−TNP complexes crystallize in 1:1 stoichiometry, whereas TAPB− mDNB as a 1:3 complex (Scheme S1, Supporting Information). All the complexes were obtained as air-stable red crystalline solids (see Figure S1, Supporting Information) and characterized by elemental analysis, NMR spectroscopy, FT-IR spectroscopy, and single crystal X-ray diffraction. 1 H NMR spectra of the complexes have been obtained from DMSO-d6 solution (see Figures S2−S5, Supporting Information). In TAPB−TNT complex, in addition to the 1H NMR signals of TAPB, signals appearing at δ 9.0 and 2.56 ppm are due to the aromatic and methyl protons of TNT, respectively. In TAPB−TNP complex, the TAPB protons shift downfield (Δδ = 0.18 ppm for Ha′, 0.19 ppm for Hb′, and 0.27 ppm for Hc′; Figure 1); the downfield shift of aromatic protons and the absence of -NH2 protons indicate the formation of −NH3+

Figure 1. Partial 1H NMR spectra of TAPB and TAPB−TNP complex, showing downfield shifting of TAPB protons upon complexation with TNP.

moieties as a consequence of proton transfer from −OH of TNP to −NH2 groups of TAPB. This phenomenon has previously been observed in many amino-trinitrophenolate complexes.16,21 Three signals at δ 8.84 (t), 8.67 (dd), and 7.48 (d) ppm correspond to mDNB in the TAPB−mDNB complex. The relative ratio of the intensities of the TAPB resonances to the PNAC analytes resonances indicate that the stoichiometry of TAPB−TNT and TAPB−TNP complexes is 1:1, while that of TAPB−mDNB complex is 1:3. Crystal Structure of the Host TAPB. TAPB crystallizes in triclinic space group P1̅ with two molecules in the unit cell. It is C

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between TAPB and TNT molecules are 3.586(3) and 3.648(2) Å, and offset distance and angle of ∼1.48 Å and ∼24°, respectively (Figure 3).24 The adjacent TAPB molecules are parallel to each other with staggered orientation. The TAPB−TNT stacks are further connected through interstack25 hydrogen bonds between -NH2 groups to form an infinite (running parallel to the stack axes) array of alternating 12(R46(12)) and 8- (R44(6)) membered rings as shown in Figure 4a,b. In the former, six nitrogen atoms are arranged in perfect chair conformation, whereas in the latter, four nitrogen atoms are arranged in a puckered form to achieve the stable conformation (Table S1, Supporting Information). One of the amino groups donates both the protons and forms bicoordinated hydrogen bonds with remaining two -NH2 groups (2.226(2) and 2.339(2) Å), and another −NH2 group accepts two protons from the neighboring -NH2 group (2.34(0) and 2.427(2) Å). These bicoordinated modes of hydrogen bonding establish the connections between 12- and 8-membered rings. The -NO2 groups of TNT lying away from the −NH2 groups, and hence N−H···O interactions are not observed as anticipated between amino and nitro groups. Weak aromatic C−H···O hydrogen bonding is observed between −NO2 groups and aromatic C−H groups (Figure 4c). Crystal Structure of TAPB−TNP. The complex TAPB− TNP crystallizes in monoclinic space group P21/n with one residue of each TAPB and TNP in the asymmetric unit. TNP shares the acidic −OH proton with one of the three −NH2 groups of TAPB. The pronated TAPB and trinitrophenolate ions are much closer at the molecular level by the virtue of π−π stacking and N−H···O, N−H···N, and C−H···O hydrogen bonding interactions (Figure 5a). Both phenolate oxygen and nitro groups act as hydrogen bond acceptors and form N−H··· O interactions with -NH2 groups, which results in a twodimensional (2D) hydrogen bonded sheet (Figure 5b). The -NH3+ moieties further form N−H···N interactions with -NH2 groups of adjacent TAPB molecules in the same 2D sheet. The 2D sheets are stacked over one another by π-stacking (3.74 and 3.81 Å) interactions to form 3D supramolecular network in which both the components are stacked in a slipped fashion with an offset angle of 29° and distance of 1.95 Å. The distances between two consecutive TAPB or two TNP molecules are almost equal (∼6.94 Å). Crystal Structure of TAPB−mDNB. TAPB−mDNB complex differs from the two other complexes described

a propeller shape molecule with dihedral angles 40−46° between the central and peripheral aryl rings. The centrosymmetry related molecules form double decker star shape dyads22 through face to face π-stacking (3.584 Å) between the ring centroids (see Figure S9, Supporting Information). These adjacent molecules pack one over the other with perfect staggering of the amino aryl rings. TAPB molecules are interconnected via intermolecular hydrogen bonds between -NH2 groups to form a three-dimensional network (Figure 2)

Figure 2. A view of crystal structure of TAPB showing intermolecular hydrogen bonds (dashed green lines).

including bicoordinated hydrogen bonding interactions (2.416 and 2.195 Å).23 The presence of such a hydrogen bonded supramolecular framework renders TAPB an ideal host for encapsulating the PNAC analytes. Intercalation of PNAC analytes with TAPB molecules forms TAPB−PNAC complexes. The electron-rich nature of TAPB rings and the electron-deficient nature of PNAC analytes govern the co-crystal formation predominantly by donor− acceptor π-stacking. Crystal Structure of TAPB−TNT. TAPB−TNT is a 1:1 complex of TAPB and TNT, crystallized in triclinic space group P1̅ with one molecule each of TAPB and TNT in the asymmetric unit of the unit cell. The structural analysis confirms that the crystals contain -A-B-A-B- type of stacks of alternating TAPB and TNT molecules running along the crystallographic a-axis. The centroid−centroid distances

Figure 3. Views of the sections of crystal structures of TAPB−PNAC complexes, (a) TAPB−TNT, (b) TAPB−TNP, and (c) TAPB−mDNB showing π-stacking. D

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Figure 4. Crystal structure of TAPB−TNT, (a) showing hydrogen bonds (green dashed bonds) and π-bonds (orange dashed lines), (b) infinite zigzag array of alternating 12- and 8-membered N−H···N hydrogen bonded rings running along the a-axis and (c) C−H···O hydrogen bonding.

Figure 5. Crystal structure of TAPB−TNP, (a) showing hydrogen bonds (green dashed bonds) and π-bonds (orange dashed lines) between 2D sheets and (b) a 2D sheets showing C−H···O and N−H···O and N−H···N hydrogen bonding interactions.

Figure 6. Packing diagrams of TAPB−mDNB complex, (a) showing TAPB and mDNB tapes connected by N−H···O hydrogen bonds (green dashed lines) and (b) a view of space fill model where the atoms are represented by spheres of van der Waals radii. TAPB and mDNB molecules are shown by gray and red colors, respectively.

This arrangement forms distinct tapes of TAPB and mDNB which are interconnected by intermolecular N−H···O bonding between -NH2 and -NO2 groups (Figure 6a). Figure 6b shows a space filling model with different colors, which clearly establishes the containment of m-DNB within the layers of TAPB. Thermal Behavior. The thermal behavior of the co-crystals was determined by DSC and thermogravimetric analysis (TGA). The DSC profiles (Figure S15, Supporting Information) reveal strong endothermic peaks at 170, 243, and 121 °C

above in terms of the stoichiometry of the complex, which crystallizes in triclinic P1̅ space group with one molecule of TAPB and three molecules of mDNB in the asymmetric unit. Close inspection of the molecular structure of TAPB−mDNB complex reveals that it is quite different from TNT and TNP complexes. In this case the intercalation of mDNB occurs between peripheral rings of TAPB to form three nonequivalent π-stacks within the range of TAPB periphery depending on the orientation as shown in Figure 3 (vide supra). The distances between the nearest rings centroids vary from 3.57 to 4.11 Å. E

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for TAPB−TNT, TAPB−TNP, and TAPB−mDNB co-crystals, respectively. These peaks are attributed to melting temperatures of the complexes. The melting points are higher than the PNAC components and lower than that of TAPB. This indicates that the melting point can be greatly influenced by cocrystallization. Strong exothermic peaks at 241, 253, and 330/ 420 °C indicate decomposition of TAPB−TNT, TAPB−TNP, and TAPB−mDNB complexes, respectively (Table S2, Supporting Information). TAPB−mDNB complex exhibits two exothermic peaks that could be ascribed to the deassembling of the complex into TAPB and mDNB components and then heterogeneous decomposition of the components as has been observed previously in BTF/mDNB (BTF = benzotrifuroxan) co-crystals.8a

CONCLUSIONS We have demonstrated here that the propeller-shaped C3symmetric 1,3,5-tris(4′-aminophenyl)benzene (TAPB) is an excellent supramolecular host for polynitroaromatic compounds such as TNT, TNP, and mDNB. The X-ray diffraction data revealed parallel stacks of vertically overlapped alternating TAPB and PNAC compounds through significantly short intermolecular π−π distances, which is ascribable to a direct charge transfer phenomenon between TAPB and PNACs. In all the three systems studied, the X-ray structure determination clearly reveals the fact that the dominant interaction between host TAPB and PNAC guest is a strong π−π interaction that is induced by the inherent electron-rich and electron-deficient nature of the host and guest, respectively. The formation of N− H···X (X = N or O) hydrogen bonds between the -NO2 and -NH2 groups further stabilize the crystal lattice. To the best of our knowledge, this is the first example of a triphenylbenzenebased system, which has been used as a host for PNACs. Further studies on development of newer triphenylbenzene systems are currently underway. ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data in CIF format, 1H, 13C NMR, FT-IR spectra, additional crystal packing diagrams. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data have also been deposited with Cambridge Crystallographic Data Centre; CCDC numbers 940123 (for TAPB), 940124 (for TAPB−TNT), 940125 (for TAPB− TNP), and 940126 (for TAPB−mDNB) and copy of which can be obtained free of charge from www.ccdc.cam.ac.uk/conts/ retrieving.html.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91 22 2576 7163. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank DST Nano Mission (No. SR/NM/NS-1119/2011), SERB (No. SR/S1/IC-12/2009) New Delhi, and DAE-BRNS (No. 2010/21/04-BRNS), Mumbai, India, for financial support. P.V. thanks CSIR, New Delhi, for a research fellowship (J.R.F. and S.R.F.). F

dx.doi.org/10.1021/cg500948h | Cryst. Growth Des. XXXX, XXX, XXX−XXX